Automobiles have undergone many changes since their initial development at the end of the last century. Many of these evolutionary changes can be seen as a maturing of technology, with the fundamental principles remaining the same. Such is the case with the ignition system. Some of its developments include the replacement of mechanical distributors by electronic ones, increasing reliability and allowing for easy adjustment of the spark timing under different engine operating conditions. The electronics responsible for creating the high voltage required for the discharge have changed, with transistorized coil ignition (TCI) and capacitive discharge ignition (CDI) systems common today. However, the basic spark plug structure has not changed. Spark plugs today differ from earlier ones mostly in the use of improved materials, but the basic point-to-point discharge remains the same.
A spark driven by the force from the interaction of the magnetic field created by the spark current and the current itself is very attractive concept, for enlarging the ignition kernel for a given ignition system input energy.
The need for an enhanced ignition source has long been recognized. Many inventions have been made which provide enlarged ignition kernels. The use of plasma jets and Lorentz force plasma accelerators have been the subject of much study and patents. None of these prior inventions have resulted in practical commercially acceptable solutions, though. The primary weakness of the prior inventions has been the requirement for excessive ignition energy, which eliminates any possible efficiency enhancement in the engine in which they are employed. These higher ignition energy requirements have resulted in high rates of ignition electrode erosion, which reduces ignition operating life to unacceptable levels.
The concept of enlarging the volume and surface area of the spark initiated plasma ignition kernel is an attractive idea for extending the practical lean limit for combustible mixtures in a combustion engine. The objective is to reduce the variance in combustion delay which is typical when engines are operated with lean mixtures. More specifically, there has been a long felt need to eliminate ignition delay, by increasing the spark volume. While it will be explained in more detail below, note that if a plasma is confined to the small volume between the discharge electrodes (as is the case with a conventional spark plug), its initial volume is quite small, typically about 1 mm.sup.3 of plasma having a temperature of 60,000.degree. K. is formed. This kernel expands and cools to a volume of about 25 mm.sup.3 and a temperature of 2,500.degree. K., which can ignite the combustible mixture. This volume represents about 0.04% of the mixture that is to be burned to complete combustion in a 0.5 liter cylinder at a compression ratio of 8:1. From the discussion below it will be seen that, if the ignition kernel could be increased 100 times, 4% of the combustible mixture would be ignited and the ignition delay would be significantly reduced. This attractive ignition goal has not heretofore been achieved in practical systems, though.
The electrical energy required in these earlier systems, e.g., Fitzgerald et al., U.S. Pat. No. 4,122,816, is claimed to be more than 2 Joules per firing (col. 2, lines 55-63). This energy is about 40 times higher than that used in conventional spark plugs.
Matthews et al., infra, reports the use of 5.5 Joules of electrical energy per ignition, or more than 100 times the energy used in conventional ignition systems.
Consider a six cylinder engine operating at 3600 RPM, which requires firing three cylinders every engine revolution or 180 firings per second. At 2 Joules per firing this is 360 Joules/second. This energy must be provided by the combustion engine at a typical efficiency of about 18% and converted to a suitable higher voltage by power conversion devices with a typical efficiency of about 40%, for a net use of the engine fuel at an efficiency of about 7.2%. Fitzgerald requires a fuel consumption of 360/0.072 Joules/second, or about 5000 Joules/second to run the ignition system.
To move a 1250 kg vehicle on a level road at about 80 km/hr (about 50 mph) requires about 9000 Joules/second of fuel energy. At an engine fuel to motive force conversion efficiency of 18%, about 50,000 Joules/second of energy will be consumed. Thus, the system employed by Fitzgerald et al, infra, will consume about 10% of the fuel energy consumed to run the vehicle to run the ignition system. This is greater than the efficiency gain to be expected by use of the Fitzgerald et al. ignition systems.
By comparison, conventional ignition systems use about 0.25 percent of the fuel energy to run the ignition system. Further, the high energy employed in these systems causes high levels of erosion to occur in the electrodes of the spark plugs, thus reducing the useful operating life considerably. This shortened life is demonstrated in the work by Matthews et al., infra, where the need to reduce ignition energy is acknowledged although no solution is provided.
As an additional attempt at solving this problem, consider the work by Tsao and Durbin (Tsao, L. and Durbin, E. J., "Evaluation of Cyclic Variation and Lean Operation in a Combustion Engine with a Multi-Electrode Spark Ignition System", Princeton Univ., MAE Report, (January, 1984)), where a larger than regular ignition kernel was generated by a multiple electrode spark plug, demonstrating a reduction in cyclic variability of combustion, a reduction in spark advance, and an increase in output power. The increase in kernel size was only six times that of an ordinary spark plug.
Bradley and Critchley (Bradley, D., Critchley, I. L., "Electromagnetically Induced Motion of Spark Ignition Kernels", Combust. Flame 22, pgs. 143-152 (1974)) were the first to consider the use of electromagnetic forces to induce a motion of the spark, with an ignition energy of 12 Joules. Fitzgerald (Fitzgerald, D. J., "Pulsed Plasma Ignitor for Internal Combustion Engines", SAE paper 760764 (1976); and Fitzgerald, D. J., Breshears, R. R., "Plasma Ignitor for Internal Combustion Engine", U.S. Pat. No. 4,122,816 (1978)) proposed to use pulsed plasma thrusters for the ignition of automotive engines with much less but still substantial ignition energy (approximately 1.6 J). Although he was able to extend the lean limit, the overall performance of such plasma thrusters used for ignition systems was not significantly better than that of regular spark plugs and the sparks they produce. In this system, much more ignition energy was used without a significant increase in plasma kernel size. (Clements, R. M., Smy, P. R., Dale. J. D., "An Experimental Study of the Ejection Mechanism for Typical Plasma Jet Ignitors", Combust. Flame 42, pages 287-295 (1981)). More recently Hall et al. (Hall, M. J., Tajima, H., Matthews, R. D., Koeroghlian, M. M., Weldon, W. F., Nichols, S. P., "Initial Studies of a New Type of Ignitor: The Railplug", SAE paper 912319 (1991)), and Matthews et al. (Matthews, R. D., Hall, as M. J., Faidley, R. W., Chiu, J. P., Zhao, X. W., Annezer, I., Koening, M. H., Harber, J. F., Darden, M. H., Weldon, W. F., Nichols, S. P., "Further Analysis of Railplugs as a New Type of Ignitor", SAE paper 922167 (1992)), have shown that a "rail plug" operated at an energy of over 6 J (2.4 cm long) showed a very substantial improvement in combustion bomb experiments. They also observed improvements in the lean operation of an engine when they ran it with their spark plug at an ignition energy of 5.5 J. They attributed the need of this excessive amount of energy to poor matching between the electrical circuit and the spark plug. This level of energy expended in the spark plug is about 25% of the energy consumed in propelling a 1250 kg vehicle at 80 km/hr on a level road. Any efficiency benefits in engine performance would be more than consumed by the increased energy in the ignition system.